This invention relates to a passive maser and a process for controlling a passive maser.
U.S. Pat. No. 4,122,408 issued to F. L. Walls discloses a system and method for achieving frequency stabilization of a passive maser so that the maser can form a frequency standard. A local oscillator provides a carrier signal which is phase or frequency modulated by a pair of modulating signals, and the multiple modulated carrier signal is then processed and the resulting probe signal coupled to the maser cavity substantially at a frequency close to the natural frequency of the atomic resonance line (e.g., hydrogen) internal to the cavity. The output signal coupled from the cavity is amplitude modulated, with the modulation at the first modulation frequency having a level proportional to any frequency offset between the probe signal and the atomic resonance line, and the modulation at the second modulation frequency having a level proportional to any frequency offset between the resonant frequency of the cavity and the frequency of the probe signal. The output signal from the cavity is processed and utimately rectified so that the various amplitude modulations on the output signal can be recovered. The error signal resulting from synchronous detection of the amplitude modulation signal of the second frequency, is coupled to the tuning element of the cavity tuning control circuitry to precisely tune the resonant frequency of the cavity to the frequency of the probe signal. The error signal resulting from synchronous detection of the amplitude modulation signal of the first frequency is coupled to the local oscillator to adjust the frequency so that the probe signal is maintained at the center of the atomic resonance line. In this manner, the resonant frequency of the cavity and the local probe signal are stabilized to the atomic resonance line, thereby also stabilizing the local oscillator.
The major shortcomings of this system are that the harmonic content of the probe modulation signal must be very low and that the timing of the servo circuits is critical. Thus, precision analog adjustments are required for setup and must be repeated at intervals throughout the life of the frequency standard. This is particularly disadvantageous when the maser is to be used as a frequency standard on an orbiting spacecraft, where adjustments of this nature are not practical.